This program for the continued development of the laboratoy rabbit as a research tool emphasizes inbreeding, the immunogenetic characterization of inbred or selectively bred animals, and the use of these animals for studies of the histocompatibility systems, transplantation of organs and as a model for tumor immunology. We continue to make available characterized animals, tissue and red cell typing service and typing reagents to all qualified investigators. Studies on the major histocompatibility system of the rabbit have now moved into the study of the cross reacting groups of antigens which are detected by serological methods and their relationship to the specificities detected by the mixed lymphocyte culture test. It appears that the serological test and MLC reaction are detecting the products of a single locus or of a pair of very closely linked genes. It is through the partitioning of the complex array of antibodies obtained by cross immunization by tissues that we hope to gain a better understanding of this animal model of the D-D loci of the human HLA system. A large variety of isoimmunization are underway and the resulting sera are being tested against subsets of lymphocytes and other formed elements of the blood in an effort to elucidate the fine structure of the rabbit major histocompatibility complex. Related to the studies of histocompatibility is our effort to describe marker genes located in the area of MHC through our study of serological or biochemical polymorphisms in the rabbit.